Ion trapping mass spectrometers utilizing magnetic confinement of the ions in the radial direction and DC voltages for axial confinement are known as Penning Traps or ion cyclotron resonance mass spectrometers (ICR-MS). Ions in the trapping cell oscillate at a frequency that depends on the magnetic field strength and the mass-to-charge (m/z) ratio of the ion. Ions trapped in the detector cell can absorb energy by resonance excitation from an applied electrical field alternating at the frequency of oscillation of the ions, and can be detected by measuring the electromotive force (EMF) induced in the trapping cell walls due to the oscillating charge of the ions by means known in the art. Fourier Transform Mass Spectrometers (FTMS) detect the masses of ions by exciting the ions in the detector cell by means of a voltage pulse containing a range of frequencies or a rapid frequency scan so as to increase the energy of all of the ions present in the cell when the excitation frequency matches the ion oscillation frequency. The detected voltage is a complex mixture of frequencies that corresponds to the natural oscillation of all of the ions that were excited. A Fourier Transform of the time domain voltage results in a frequency domain spectrum that directly represents the mass and relative abundances of the ions present.
Ions are generally formed in an ion source located outside of the magnetic field and must be accumulated in an ion trapping device and then transported into the detector cell and in the magnetic field. Since there is no inherent means of increasing the number of charged particles that are detected when detecting ions by induced EMF, as is common in other types of mass spectrometers which utilize electron multipliers, it is necessary to have a large-volume detector cell that can hold several million ions. Typically at least 100 ions are required for a minimum detectable voltage. It is known in the art to accumulate ions in a radio frequency (RF) ion trap comprising a multipole electrode structure, such as a hexapole or octopole, having RF voltages applied to the electrodes to confine the ions in the radial direction. DC voltages applied to apertures located on the axis of the accumulation trap and at the entrance and exit ends of the trap confine the ions in the axial direction.
FIG. 1 is a schematic view of a typical FTMS system 100. In this schematic view, ions travel in a general direction from left to right along an axis about which various ion-controlling devices are arranged. The FTMS system 100 generally includes an ion source (not shown) followed by, in succession along the axis, an ion accumulator 102, a shutter assembly 104, an ion guide 106, an ion decelerator 108, and an ion detector cell 110. The FTMS system 100 also includes a housing 112 that encloses the ion accumulator 102, the shutter assembly 104, the ion guide 106, the ion decelerator 108 and the ion detector cell 110. The housing 112 defines a first vacuum region (or pumping stage) 114 and a second vacuum region (or pumping stage) 116 adjoined at a boundary 118 having a differential pumping aperture 120 located at the axis. The ion accumulator 102 and the shutter assembly 104 are positioned in the first pumping region 114 and the ion guide 106, the ion decelerator 108 and the ion detector cell 110 are positioned in the second pumping region 116. Suitable vacuum pumps 122, 124 respectively maintain the first vacuum region 114 at a vacuum pressure P1 and the second vacuum region 116 at a vacuum pressure P2 lower than P1. The FTMS system 100 further includes a suitable magnet assembly 126 (e.g., including a superconducting magnet) that coaxially surrounds the ion detector cell 110 and may also surround the ion decelerator 108 and part of the ion guide 106.
FIG. 2A is a side (lengthwise) view of the ion accumulator 102, shutter assembly 104 and ion guide 106 illustrated in FIG. 1. The ion accumulator 102 and the ion guide 106 are typically structured as linear multipole electrode sets operating as ion traps. Each electrode set includes a set of parallel electrodes 232, 234 extending along the axis and circumferentially spaced from each other about the axis at radial distances in the transverse plane orthogonal to the axis, thereby circumscribing an axially elongated interior space in which ions may be confined and through which the ions travel. Typically, each electrode set includes six electrodes 232, 234 (hexapole arrangement) or eight electrodes 232, 234 (octopole arrangement). RF voltage sources (not shown) are connected to the electrodes 232, 234 in a known manner so as to apply a linear (two-dimensional) RF trapping field that confines the radial motions of the ions to a region along the axis. Respective lenses 236, 238 serve as the ion entrance to and ion exit from the ion accumulator 102. Another lens 242 serves as the ion entrance to the ion guide 106 and yet another lens (not shown) serves as the ion exit from the ion guide 106. The lenses 236, 238, 242 are typically plates with apertures located at the axis and are connected to DC voltage sources (not shown). The shutter assembly 104 is typically a series of lenses 244 configured to direct the ions through the differential pumping aperture 120 located between the two vacuum regions 114 and 116 (FIG. 1). The shutter assembly 104 also typically includes a movable, mechanical shutter element (not shown). As an alternative to an RF multipole arrangement, the ion guide 106 may be provided as a series of axially spaced DC lenses that would likewise operate to confine the ions in the radial direction as the ions travel to the ion detector cell 110.
In operation, ions 248 produced from a molecular sample in the ion source are transmitted in the ion accumulator 102. In the ion accumulator 102, the ions are confined in the radial direction by the RF voltages applied to the electrodes 232 and in the axial direction by the DC voltages applied to the entrance lens 236 and the exit lens 238. FIG. 2B illustrates typical DC voltages applied to the ion accumulator 102, shutter assembly 104 and ion guide 106 when trapping ions in the ion accumulator 102. Assuming the ions are positively charged, a positive DC voltage (e.g., +5 V) is applied to the entrance lens 236, no DC voltage is applied to the electrodes 232 of the ion accumulator 102, a relatively higher DC voltage (e.g., +20 V) is applied to the exit lens 238, and a negative DC voltage (e.g., −7 V) is applied to the electrodes 234 of the ion guide 106. The low potential barrier at the entrance to the ion accumulator 102 allows the ions to enter the ion accumulator 102. The large potential barrier at the exit of the ion accumulator 102 prevents ions from passing completely through the ion accumulator 102 while the ions are being accumulated therein. The addition of a damping gas such as helium allows for the removal of excess kinetic energy by collisions so that the ions will not escape from the ion accumulator 102 by leaving through the aperture of the entrance lens 236.
FIGS. 3A and 3B illustrate the extraction of the ions from the ion accumulator 102. FIG. 3A is a side (lengthwise) view of the ion accumulator 102, shutter assembly 104 and ion guide 106 similar to FIG. 2A, and FIG. 3B illustrates typical DC voltages applied to the ion accumulator 102, shutter assembly 104 and ion guide 106 when extracting the trapped ions from the ion accumulator 102. Ions are removed from the ion accumulator 102 by reducing the potential barrier at the exit lens 238, for example by changing the DC voltage on the exit lens 238 from +20 V to −20 V as shown in FIG. 3B. Additionally, in prior art devices a large number of ions are accumulated so as to form space charge repulsion between the ions. The space charge repulsion, along with the attractive potential from the exit lens 238 of the ion accumulator 102, causes ions to be removed from the ion accumulator 102 and directed through the shutter assembly 104 and into the ion guide 106. During ion extraction from the ion accumulator 102, the shutter element of the shutter assembly 104 opens to allow ions to pass and closes after the ions have passed in order to reduce the gas load on the vacuum pump 124 in the second pumping region 116 (FIG. 1), thereby allowing lower pressures to be maintained during the succeeding mass analysis time. After traversing the differential pumping aperture 120 (FIG. 1), the ions then travel through the ion guide 106. Ions 250 exiting the ion guide 106 are decelerated and transmitted into the magnetic field and into the ion detector cell 110.
FIG. 4A is a side (lengthwise) view of the ion decelerator 108 and ion detector cell 110 illustrated in FIG. 1, as well as part of the ion guide 106 preceding the ion detector cell 110. The ion detector cell 110 typically includes three axially spaced electrodes 454, 456, 458 (cylindrical rings or plates) with respective apertures aligned along the axis, and trapping plates 108, 462 positioned at the respective axial ends. The trapping plate 108 at the ion entrance is typically a lens with an aperture, and typically serves as the ion decelerator 108. The center electrode 456 is further segmented into radial quadrants (not shown) so as to have pairs of opposing sections that can be utilized as transmitting and receiving electrodes for ion detection and mass measurement. In addition to applying alternating frequency voltages to the electrodes 454, 456, 458 for ion detection, each electrode 454, 456, 458 can also have a DC potential applied thereto. FIG. 4B illustrates typical DC voltages applied to the various electrodes of the ion guide 106, ion decelerator 108 and ion detector cell 110 when admitting ions in the ion detector cell 110, and also schematically illustrates the trajectory of the ions during this time. A negative DC voltage (e.g., −7 V) is applied to the electrodes 234 of the ion guide 106 as noted above, no DC voltage is applied to the ion decelerator 108, a positive DC voltage (e.g., +0.2 V) is applied to the first inner electrode 454, no DC voltage is applied to the center electrode 456, a positive DC voltage (e.g., +0.2 V) is applied to the second inner electrode 458, and a positive DC voltage (e.g., +15 V) is applied to the distal trapping plate 462. The voltage at the distal end of the ion detector cell 110 has a repulsive DC potential applied to prevent the in-coming ions from escaping the detector cell 110 at that end, as indicated schematically by the ion trajectory in FIG. 4B. Ions are confined in the radial direction by the magnetic field. The potential at the entrance (proximate) end of the ion detector cell 110 is reduced so as to allow ions from the accumulator trap 102 to enter the detector cell 110 similar to what was described above for the accumulator trap 102. Once the packet of ions has entered the ion detector cell 110, the potential at the entrance is increased so as to prevent the ions in the detector cell 110 from escaping from the entrance end. This is shown in FIGS. 5A and 5B. FIG. 5A is a side (lengthwise) view of the ion decelerator 108, ion detector cell 110 and part of the ion guide 106 similar to FIG. 4A, and FIG. 5B illustrates typical DC voltages applied to the ion guide 106, ion decelerator 108 and ion detector cell 110 when trapping the ions in the ion detector cell 110. FIG. 5B also schematically illustrates the trajectory of the ions during this time. The large potential barrier at the entrance to the ion detector cell 110 is accomplished by changing the DC voltage on the decelerator 108 from 0 V to +15 V.
Significant drawbacks are associated with conventional FTMS systems such as described above and illustrated in FIGS. 1-5B. Ions traveling towards the detector cell 110 from the accumulator trap 102 begin to spread in space and time due to the differences in their masses and velocities. A further spreading of ions of the same mass will occur due to the energy variation of the ions due to the initial conditions and distribution of electric fields utilized to remove the ions from the accumulator trap 102. Because of the spread of the ions in space and time it is difficult to efficiently transport ions of a large mass range into the detector cell 110. Moreover, the reliance on the combination of ion space charge and a voltage differential between the accumulator trap 102 and the exit aperture 238 causes a variable and highly non-linear ion extraction field that further degrades the efficiency and the mass range of ions capable of being trapped in the detector cell 110. Furthermore, the electric field formed from the space charge changes as charge is removed from the accumulator trap 102. Space charge forces are a function of mass in addition to the number of charges and their spatial distribution. Furthermore, a decelerator 108 in the form of a single lens at the entrance to the detector 110 cannot produce a uniform electric field both along the axis and off the axis, but rather the field will be non-uniform, i.e. the strength (V/mm) of the field will not be constant.
In view of the foregoing, there is a need for more efficient methods and means for transporting ions from the accumulator trap into the detector cell. There is also a need for methods and apparatus that allow a larger mass range of ions to be simultaneously transported and trapped in the detector cell.